1. Field of the Invention.
The present invention relates generally to telecommunication systems and, more specifically, to radio transmission systems for communication between two or more telecommunication units, at least one of which can be mobile.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
In recent years various techniques and concepts of private radio communication systems for mobile telephony and data transmission have been developed. In general two types of mobile communication systems can be distinguished.
The first is cellular communication, providing coverage over a relatively wide area, i.e. relatively large cells. Analogue cellular systems, such as designated AMPS, ETACS, NMT-450 and NMT-900 have been deployed throughout the world. Digital cellular systems are designated IS-54B in North America and the pan-European GSM system.
The second is cordless communication, ranging from simple residential cordless telephones to business cordless systems capable of serving hundreds or even thousands of cordless communication units across (large) offices, production halls etc. Analogue cordless telephones are designated CT0, CT1 and CT1+. Amongst the digital cordless systems, designated CT2, CT2-CAI, CT3 and DECT (Digital Enhanced Cordless Telecommunications), both CT3 and DECT use TDMA (Time Division Multiple Access) as their transmission technique, whereas CT2 operates under FDMA (Frequency Division Multiple Access). Besides these access techniques, in particular in North America, spread spectrum access is used for cordless communication. CDMA (Code Division Multiple Access) is another digital access technique which can be used for cordless communication.
At present, for each of the above technologies, specific radio frequency bands have been allocated. These radio frequency bands are sub-divided into various fixed radio channels. In CT3 and DECT systems, for example, is each radio channel further sub-divided in time into a number of communication channels. The radio channels of the various cordless technologies have a different frequency bandwidth, mainly due to a different number of communication channels in one radio channel and the modulation technique used. The bandwidth of each radio channel equals at least the total communication bandwidth, i.e. the number of communication channels in a particular radio channel times the frequency bandwidth of a single communication channel.
Some of the cordless technologies can also be used to provide a type of Personal Communication Services (PCS) or also called Cordless Terminal Mobility (CTM). This is basically a mobile telephone like service that covers a whole town or city. Customers of this service are able to carry their mobile communication units (telephone handsets) with them and can make calls in the coverage area of the service.
As deregulation gathers pace in telecommunication multiple operators will enter the market, using different radio technologies to provide some kind of PCS service in a given area. In such a case, one of the problems which has to be solved relates to the mutual isolation between the various communication systems and/or technologies used by the different operators, in order to avoid disturbances.
Mutual isolation could be provided by assigning special frequency bands or radio channels to specific operators, prohibiting the use thereof by others, for example. However, this is a very inefficient manner of using the available scarce radio frequency bandwidth. This, because it is difficult to estimate beforehand the required bandwidth for each operator. Further, it is not likely that the several operators will use their available transmission capacity to the same extents such that a lot of bandwidth is not efficiently used. It is even likely that the different operators will only set up a service in one or a few geographical areas, which involves a complex planning of bandwidth and radio channel allocation. To allow for new technologies and an increase of the service, an amount of frequency bandwidth has to hold in reserve, with the risk that it will not be used after all.
Instead of dedicated frequency bands or radio channels for each of the operators, another option would be the allocation of special PCS frequency bands or radio channels for each of the different cordless technologies, for example. However, in case several operators would like to cover a certain area with the same technology, such as DECT, it is necessary to operate the various systems in a synchronised manner.
Accordingly, for providing a PCS service, or in general any other (public) mobile radio telecommunication service, it would be very attractive to allow for multiple independent operated radio communication systems of the same or different technology to coexist in one or a few common radio frequency bands.
To provide for such a service, in which the various communication systems are allowed to operate essentially over a common radio frequency band, amongst others, access rules at the start of the transmissions have to be defined. Such that new transmissions, which would cause harmful disturbances to transmissions in progress, are prohibited to access part or parts of the radio frequency band temporarily occupied.